[ET-VK][Ez] Allow ParamsBindList to append a single Binding Info (#4636)

## Context

This diff adds some API functions to `ParamsBindList` to make it easier to use, specifically

1. Added default constructor
2. Added overload for `append` that takes only one `BufferBindInfo`

The reason for these changes is to make the following pattern easier:

```
ParamsBindList ubo;
if (kernel1) {
  ubo.append(ubo1);
}
else {
  ubo.append(ubo2);
}
```

This pattern was not possible before because `ubo` could not be default constructed, and `ubo1` and `ubo2` had to be wrapped in an initializer list before being passed to `append`.
Differential Revision: D60930997

Pull Request resolved: https://github.com/pytorch/executorch/pull/4593

---------

Co-authored-by: Stephen Jia <ssjia@meta.com>